This argument could be made for/against towing at the limit, regardless of whether there is excess capacity in the TV or not.
huh? Once again you have lost me. How can this argument be made for/against towing at the limit if you don't have excess capacity in the tow vehicle? The argument is moot if you stay under the limit.
a. What are the limits/ratings?
Those are determined by the industry, either through some third party, i.e. SAE, or proprietary. Stay within them.
b. Should one adopt some form of safety margin to those limits/ratings (and stay away from them).
This point is also made moot if you stay under the limits. You don't have to adopt your own safety limits if you stay within guidelines. Better yet, how about employing some common sense. If I'm going to be pulling a trailer cross country, maybe it would be a good idea to NOT be at max capacity. Seems a sensible self-imposed safety margin to me. But since you ask, there are some that belong to camp that apply "the 80% rule". That is for maximum safety, stability and as not to over tax a tow vehicle during towing, staying within 80% of tow rating is prudent to THEM. I personally think 80% is an arbitrary number. I prefer to gauge my tow vehicle's performance against conditions, load, distance, etc. I have a more than capable tow vehicle that is rated for 7100lbs towing but I would only tow that much under the right conditions because I know how my tow vehicle handles when I'm tugging my 5700lb rig.
I really don't want to muddy the waters by going on that tangent.
Why stop now? All you have done is muddy the waters by sharing your "standards" and opinions, i.e. let's use creative math to eek out ever ounce of capacity that we can, ignoring previously established standards and rules, ignore the extra passengers and cargo in the tow vehicle when you are at max tow rating, attributing comments to me that I never made, claiming that I clarify your point when actually I was just describing a trend in the auto industry, making light of the fact that other sources of authority other than an owner's manual might be out there to better understand the task of towing.
I think we have both stated our points, and we disagree with each other. And that's fine. That's what these forums are great for. Let people state different opinions, and the OP can sort it thru for themselves.
There certainly have been a bevy of opinions shared here but in my case, I use FACTS whenever possible to dispute your OPINION. The FACT is you must deduct the weight of extra passengers and cargo in the tow vehicle from the max tow rating. In your OPINION, you think you can ignore this long standing industry standard because crossovers are "structure-limited" (whatever that means). Don't believe me? Maybe this sticky posted by our very own moderators will change your mind:
http://www.explorerforum.com/forums/showthread.php?t=176988
This is your max towing capacity. Meaning all cargo, passengers and weight of trailer and contents included. Don't think you can load up with stuff in the truck and still tow this amount of weight on a trailer!
Oh wait that sticky only contains ratings for previous generations before Ford made Explorers crossovers (i.e. "structure-limited?"). Ok, so take a look at other auto makers websites and I'd be willing to bet that you would find cases where they state to deduct passengers/cargo from tow rating even for crossovers.
Oh wait, those only apply to other brands, not Ford? Ok, then pull up a Ford towing guide (i.e.
http://www.fleet.ford.com/resources/ford/general/pdf/towingguides/14FLRVTTgde_Sep9.pdf) and it is peppered with references that include only a 150lb driver in the tow rating and to deduct additional passengers/cargo. Oh wait, that only applies to Ford's truck line? Ok, then look in the section entitled "Know your limits" and under "Maximum Loaded Trailer Weight" and it states to deduct additional passengers and gear.
Clearly, these are not my OPINIONS.
Oh wait, what about the owner's manual? Well, ya got me there. I guess despite all the FACTS that are square in my face, I'm left with just my OPINION after all.
Where are your sources?